Erosion-resistant ceramic material, powder, slip and component

ABSTRACT

The use of magnesium oxide, reactive alumina and aluminium oxide as a base provides for a new erosion-resistant material upon sintering.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT Application No. PCT/EP2017/078718, having a filing date of Nov. 9, 2017, which is based on German Application No. 10 2016 224 443.4, having a filing date of Dec. 8, 2016, the entire contents both of which are hereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to an erosion-resistant ceramic material, a powder, slip and a component.

BACKGROUND

Ceramic heat shields (CHS) as example of components made of ceramic material display corrosion and erosion on the hot gas side during use. This process is due to the corrosion of the mullite present in the CHS material, which is converted into secondary α-alumina on contact with the hot gas. This secondary α-alumina has a lower mechanical strength than the surrounding CHS material. This secondary α-alumina is ablated by the hot gas stream and the larger microstructural constituents of the component composed of the ceramic are exposed. When these microstructural constituents are exposed to a certain degree, they are detached from the CHS surface.

The ceramic heat shields CHS in the CHS series of the combustion chamber which are most affected by corrosion and erosion are often given an aluminum oxide coating on the hot gas side. This coating is applied by a slip spraying process or a flame spraying process to the CHS.

These coatings often have a relatively fine-grain structure which in plant operation tends to suffer from after-sintering, crack formation and premature disintegration.

A flame coating, on the other hand, is relatively dense, brittle and cannot follow the deformations of the CHS in plant operation. The consequences here are also crack formation in the coating and detachment of the coating constituents, partly due to the relatively poor adhesion of the flame coating to the CHS base material. The life of these coatings is relatively limited. The time for which the CHS base material is afforded protection against hot gas corrosion is thus significantly limited. The detached coating itself represents an additional source of particles which are accelerated in the direction of the turbine and can there cause damage to the turbine blade TBC.

SUMMARY

The ceramic comprises at least (in % by weight): in particular consists of, aluminum oxide as matrix material, in particular in an amount of from 92.0% to <99.0%, and mullites, in particular in a proportion of 8.0%-1.0%.

The term ceramic is used here as a general generic term and encompasses every embodiment as blank, green body, powder, slip, end product, massive or as layer.

α-Alumina or tabular aluminas are used as aluminum oxide.

The ceramic contains reactive alumina as aluminum oxide in order to reduce the water content and in order to improve the processability is present in a ceramic slip.

Preference is given to no silicon oxide and/or no silicon compounds being present in the ceramic or powder.

The powder at least comprises reactive magnesium oxide (MgO) in an amount of from 0.1% to 4.0% in order to form spinel (MgAl₂O₄) with the aluminum oxide present and from 96.0% to 99.9% of aluminum oxide.

The powder contains γ′-aluminum oxide or α-alumina or tabular aluminas as aluminum oxide or contains reactive alumina as aluminum oxide as additives for reducing the water content and for improving the processability in a ceramic slip, in particular in a proportion of from 10% by weight to 25% by weight.

The tabular alumina has at least three different particle size fractions and the tabular aluminas have a maximum particle size of up to 10 mm.

The reactive alumina has at least two different particle size fractions.

Here, the term particle size fraction refers to a powder fraction having a Gaussian or Maxwell or similar distribution. Different particle size fraction means that the particle size distributions differ significantly.

The reactive magnesium oxide (MgO) has a citric acid activity of from 10 seconds to 250 seconds.

The slip comprises at least one liquid, in particular water, and a powder as described above.

A component is produced from the ceramic, the powder or the slip.

Mullite as corrosion-prone component in the material for ceramic heat shields is, in particular, avoided completely in the new material. The remaining α-alumina in the present material is significantly more stable to hot gas. The previously observed corrosion of mullite and the associated formation of mechanically unstable secondary α-alumina therefore does not occur. The ablation of material from this ceramic is thereby reduced to the far lower corrosion and erosion of the α-alumina.

The formation of an alternative binder phase in the ceramic is achieved by addition of a small amount of reactive MgO (from 0.1% by weight to 4.0% by weight). In the production of a component composed of the ceramic, this reactive MgO functions as temporary binder by formation of Mg(OH)₂. In the firing process, magnesium oxide, which is made available by Mg(OH)₂, reacts with finely particular aluminum oxide from the remaining mix to form spinel. This spinel compound (MgAl₂O₄) replaces the previous mullite binding in the finished component. A two-phase system (α-alumina and spinel) is again achieved as a result of the spinel formation. These two-phase systems feature an improved temperature change resistance due to the slightly different coefficients of thermal expansion of the individual phases and the resulting microcracks in the microstructure.

For the ceramic, reactive alumina is used in order to reduce the water content and to improve the processability. A further positive effect brought about by this reactive alumina is the very fine pore distribution in the ceramic structure achieved thereby. While the total porosity in a component composed of the ceramic is at about the same level as in other ceramic materials, the typical average pore diameter here is, at <5 μm, significantly smaller than in the case of other cast CHS materials (typically from 5 μm to 20 μm). This fine porosity likewise has a positive effect on the thermal shock behavior of the ceramic as massive component. This reactive alumina is used in a proportion of from 10% by weight to 25% by weight in the total mix.

The remaining raw materials of the powder consist of tabular aluminas of various particle size fractions up to a maximum particle size of up to 10.0 mm. The particle size distribution (=1 fraction) of reactive alumina, binder (reactive MgO) and other mixed constituents (tabular aluminas) is matched in such a way that firstly a sufficient flowability and thus processability is achieved in the production of the material but secondly the required strength and thermal shock resistance of the material for operation in a gas turbine is also achieved.

The ceramic is, due to the absence of a proportion of mullite, significantly more stable to hot gas and thus more insensitive to corrosion and erosion than all other CHS materials used at present.

The porosity of a component composed of the ceramic or of the powder is optimized in the direction of significantly more and finer pores, as a result of which the thermal shock resistance is significantly improved, by use of a dispersing alumina.

The use of reactive MgO functions as temporary binder phase in the production of the CHS. In this way, it is possible to dispense with the use of other binders which in the future finished product could lead to adverse accompanying phenomena.

The reactive MgO forms a durable and hot-gas-stable spinel phase during firing of the ceramic. This spinel phase forms a durable bond between the relatively coarse mixed constituents in the fine-grain matrix.

Thanks to its spinel formation, the fine porosity and the other microstructure, α-alumina achieves, as only previously known refractory material without a proportion of mullite, sufficient thermal shock resistance in order to attain the required strength values in a standard test series on a hot HCF test stand (simulation of the thermal and mechanical stresses on a CHS during plant operation).

Due to the reduced corrosion and erosion of CHS composed of α-alumina compared to CHS made of other CHS materials, the life in erosion-prone regions of the combustion chamber is significantly lengthened.

The replacement rate of the ceramic heat shields due to ablation of material decreases considerably, the life of the CHS increased.

The times for the required CHS replacement in these CHS series thus decreases, as a result of which the outage time for the total plant can also be shortened.

Due to the significantly reduced ablation of material from the CHS surface, fewer particles which could have erosive effects on the turbine blade coating are carried in the direction of the turbine. This significantly increases the life of the turbine blade TBC. Significantly longer operating times for turbine blades are made possible.

The material is suitable for all applications in which a refractory material is subjected to thermal shock stresses and additionally has to withstand hot gas corrosion.

Although the invention has been illustrated and described in greater detail with reference to the preferred exemplary embodiment, the invention is not limited to the examples disclosed, and further variations can be inferred by a person skilled in the art, without departing from the scope of protection of the invention.

For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. 

1. A powder comprising (in % by weight): aluminum oxide in an amount of from 92.0% to <99.0%; and spinel in a proportion of 8.0%-1.0%.
 2. A powder comprising (in % by weight): from 96.0% to 99.9% of aluminum oxide; and reactive magnesium oxide in an amount of from 0.1% to 4.0%, to form spinel with the aluminum oxide present.
 3. The powder as claimed in claim 1, which contains γ′-aluminum oxide.
 4. The powder as claimed in claim 1, which contains α-alumina as aluminum oxide.
 5. The powder as claimed in claim 1, which contains tabular aluminas as aluminum oxide.
 6. The powder as claimed in claim 1, which contains reactive alumina as aluminum oxide as an additive for reducing a water content and for improving a processability in a ceramic slip, in a proportion of from 10% by weight to 25% by weight.
 7. The powder as claimed in claim 5, wherein the tabular alumina has at least three different particle size fractions.
 8. The powder as claimed in claim 6, wherein the reactive alumina has at least two different particle size fractions.
 9. The powder as claimed in claim 5, wherein the tabular aluminas have a maximum particle size of up to 10 mm.
 10. The powder as claimed in claim 2, wherein the reactive magnesium oxide has a citric acid activity of from 10 seconds to 250 seconds.
 11. A ceramic produced using a powder as claimed in claim 1, comprising at least (in % by weight): aluminum oxide as matrix material in an amount of from 92.0% to <99.0%; and spinel in a proportion of 8.0%-1.0%.
 12. The ceramic as claimed in claim 11, wherein aluminum oxide is present as α-alumina.
 13. The ceramic as claimed in claim 11, wherein aluminum oxide is present as tabular aluminas.
 14. The ceramic as claimed in claim 11, wherein aluminum oxide is present as reactive alumina in order to reduce a water content and to improve a processability in a ceramic slip.
 15. The powder or ceramic as claimed in claim 1, which comprises no silicon oxide and/or no silicon compounds.
 16. A slip comprising at least a liquid and a powder as claimed in claim
 1. 17. A component comprising a ceramic as claimed in claim 11 or produced from a powder or a slip.
 18. The component as claimed in claim 17, wherein 90% of all pores are smaller than 5 μm.
 19. The component as claimed in claim 17 comprising aluminum oxide and spinel. 